Process for the preparation of linear low density polyethylene

Details Process for the preparation of linear low density polyethylene Process for the preparation of linear low density polyethylene

1993-03-05

1995-02-07

Schofer, Joseph L.

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

525240, 525247, 525270, 525322, 525324, C08L 2316

Patent

active

053877497

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for the preparation of linear low density polyethylene (LLDPE) endowed with improved processability by gas phase polymerization of the monomers, in two or more fluidized bed or mechanically stirred bed reactors, wherein, whatever the order, in one of the reactors mixtures of ethylene and an alpha-olefin CH.sub.2 .dbd.CHR (R=alkyl having 1-10 carbon atoms) are polymerized to yield LLDPE and in another reactor mixture of propylene-and an alpha-olefin CH.sub.2 .dbd.CHR', where R' is an alkyl radical having 2-18 carbon atoms, are polymerized, using the same catalyst in both reactors.
LLDPE has a variety of applications, but it is particularly employed in the preparation of films, because LLDPE films are endowed with improved mechanical and optical properties compared to films of LDPE.
The production of LLDPE films, however, shows some difficulties, mainly due to the fact that the polymer in the melted state has an insufficiently high melt strength, while its viscosity in the melted state is rather high.
In order to keep the productivity unaltered, it is necessary to modify the film extruders, for instance by widening the slit or increasing the temperature of the extruder head.
These modifications cause difficulties in the cooling of the bubble being blown at the extruder outlet and dishomogeneity in the film thickness.
In addition to these drawbacks, the hot welding of LLDPE films shows poor resistance to heat.
In order to overcome the shortcomings above, it was proposed to use blends of LLDPE with-a semicrystalline copolymer of propylene with an alpha-olefin CH.sub.2 .dbd.CHR' where R' is an alkyl radical having 2-10 carbon atoms, in particular 1-butene (U.S. Pat. No. 4,871,813).
The copolymer contains from 7 to 40% by weight of alphaolefin, has a fusion enthalpy lower than 75 J/g and is added in an amount of from 1 to 25% by weight.
The LLDPE-copolymer blend is prepared by mixing in the melted state the components pre-mixed in the solid state (in the form of powder or granules).
Mixing of the components in the solid state and feeding of such mixture directly into the extruder to mold the finished article is also contemplated.
The LLDPE is obtained by conventional polymerization methods, whereas the propylene-alpha olefin copolymer is prepared separately, employing stereospecific catalysts capable of yielding a semicrystalline copolymer having a fusion enthalpy not higher than 75 J/g.
The preparation of the above blends requires two separate polymerization lines for producing the two polymer components and then a blending step for mixing the components in the melt in an extruder. Blending of polymers in the melt is a high energy-consuming operation.
It has now been found that it is possible to produce blends of LLDPE and a propylene-alpha olefin CH.sub.2 .dbd.CHR' copolymer endowed with improved processability and capable of forming films exhibiting good mechanical and optical properties directly in polymerization, using at least two reactors in series, wherein, whatever the order, in one of the reactors the LLDPE and in the other the propylene-alpha olefin copolymer are synthesized and wherein in both reactors the same catalyst is employed.
The use of the same catalyst in the various reactors in series has the advantage of utilizing a single production line instead of two, as well as growing the polymer blend onto each catalyst particle, thus obtaining a composition in which the components are homogeneously mixed in the solid state, with no need of the pelletization step which is necessary when the components are prepared by separate processes.
Therefore, the polymer can be directly fed into the film extruders, thereby attaining a higher film production rate and a lower energy consumption. In fact, the non-pelletized granules, not having been subjected to the melting-soldification process of the pelletization step, have lower crystallinity and higher melt index.
Moreover, the homogenization at the level of each single particle achievable by the process of the presen

REFERENCES:
patent: 4414132 (1983-11-01), Goodall et al.
patent: 4415718 (1983-11-01), Miyoshi et al.
patent: 4454306 (1984-06-01), Fujii et al.
patent: 4525547 (1985-06-01), Kato et al.
patent: 4567155 (1986-01-01), Tovrog et al.
patent: 4871813 (1989-10-01), Senez
patent: 4946816 (1990-08-01), Cohen et al.
patent: 4977210 (1990-12-01), Kerth et al.
patent: 5149738 (1992-09-01), Lee et al.
patent: 5166268 (1992-11-01), Ficker

Affiliated with

Also associated with

Rating

Process for the preparation of linear low density polyethylene does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Process for the preparation of linear low density polyethylene, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for the preparation of linear low density polyethylene will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-1111906